When your best intentions go south, new research suggests that it wasn’t the devil that made you do it. It was your brain.

Will power, the study found, is a finite resource, one that can be easily depleted. Which is why, when faced with a “do-I-or-don’t-I” kind of decision, you might find it easier to do the right thing when you haven’t already used up your reserve of self-control by forgoing that extra cocktail earlier in the night.

Oddly reassuring, right?

The study, to be published January 2013 in the Journal of Consumer Psychology, suggests that self-control is not a muscle that grows stronger with exercise, but is more like a finite pool. And when it gets drained, say, by saying no to that delightful little black dress you just tried on but don’t necessarily need, you’re more likely to lose your cool when that guy in the silver SUV cuts you off on the freeway driving home.

Or, for that matter, to have that second glass of Malbec once you get there.

The study, co-authored by William Hedgcock, assistant professor of marketing at the Tippie College of Business at the University of Iowa, used brain imaging to scan people as they performed self-control tasks. The fMRIs found that the brain consistently fired at full speed when tasked with recognizing a temptation, but once the part of the brain that actually manages self-control took over, it fired with less intensity the longer it continued to fight the good fight.

“Our results suggest self-control can be diminished by use,” Hedgcock tells us. “People have a hard time resisting temptation after prior acts of self-control. This can negatively affect people’s ability to maintain attention, resist tempting snacks, and resist purchasing on impulse.”

Call it good-girl fatigue? Your brain may have no trouble recognizing the naughty options, but the more you try to fight them, the harder it can be to do what’s right. Or why, as the authors write, when we work hard to pass up a second helping of lasagna, we might end up taking two pieces of cake at dessert. And while the study found no significant gender differences, writes the study’s co-author Kathleen Vohs, associate professor of marketing at the University of Minnesota, in an email from Amsterdam, “men and women often do adopt different goals (e.g., to lose weight for women [or to] pursue financial or sexual rewards for men) but they do so in similar ways.”

What’s hugely important about the study, writes Vohs, is that “we haven’t had a good understanding of how the brain changes when people engage in self-control. Which means being able to ultimately understand self-control in new ways.”

For years, researchers have known that there’s some significant work going on upstairs when we try to make decisions (whether or not those choices involve resisting temptation) about anything from buying a new pair of jeans to figuring out what to do with our lives. Many studies we’ve referenced in our book and our blog, in fact, have shown that any kind of mental overload can mess with our ability to make good choices — or any at all. A pivotal study by Harvard psychologist George Miller back in the 1950s found that the rational brain can only hold about seven chunks of information in working memory at any given time. Any more, and the conscious brain often just throws up its hands in defeat. What psychologists have figured out since, is that when the cognitive brain gets too full, decision making—if it gets done at all—gets appropriated by the emotions. Because the rational brain has, in effect, logged off, there’s less control over gut impulses.

Some fifty years after Miller’s essay, Stanford marketing professor Baba Shiv put an interesting spin on this epic battle between heart and mind. In his study, he asked students to memorize either seven digits or two, then afterward offered both groups their choice of a reward: fruit salad or gooey chocolate cake. What he found was that the seven-digit group went with their gut instincts. They overwhelmingly chose cake. The two-digit folks? They made the rational decision and chose a snack that wouldn’t spoil their dinner: nice and healthful fruit salad.

A number of other studies have also shown that making choices just plain wears us out, which brings us back to some earlier work on self-control by Kathleen Vohs. Back in 2008, she found that folks who are faced with too many choices have a tough time staying focused or exerting self-control afterward. In one part of that study, she found that college students asked to make a number of random choices earlier in the day ended up spending more time later playing video games or reading magazines — rather than studying for a test.

But back to this particular study, Hedgcock suggests that when the well of self-control runs dry, the only way to fill it back up is with time. In other words, give yourself a break.